Catalytic domain of Cyclin-Dependent protein Kinase 1-like Serine/Threonine Kinases. Serine/Threonine Kinases (STKs), Cyclin-Dependent protein Kinase 1 (CDK1)-like subfamily, catalytic (c) domain. STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. The CDK1 subfamily is part of a larger superfamily that includes the catalytic domains of other protein STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase. CDKs belong to a large family of STKs that are regulated by their cognate cyclins. Together, they are involved in the control of cell-cycle progression, transcription, and neuronal function. This subfamily is composed of CDK1 from higher eukaryotes, plants, and yeasts, as well as CDK2 and CDK3. CDK1 is also called Cell division control protein 2 (Cdc2) or p34 protein kinase, and is regulated by cyclins A, B, and E. The CDK1/cyclin A complex controls G2 phase entry and progression while the CDK1/cyclin B complex is critical for G2 to M phase transition. CDK2 is regulated by cyclin E or cyclin A. Upon activation by cyclin E, it phosphorylates the retinoblastoma (pRb) protein which activates E2F mediated transcription and allows cells to move into S phase. The CDK2/cyclin A complex plays a role in regulating DNA replication. Studies in knockout mice revealed that CDK1 can compensate for the loss of the cdk2 gene as it can also bind cyclin E and drive G1 to S phase transition. CDK3 is regulated by cyclin C and it phosphorylates pRB specifically during the G0/G1 transition. This phosphorylation is required for cells to exit G0 efficiently and enter the G1 phase.